The present invention relates generally to a stray light correction method for imaging light and color measurement.
Charge coupled devices (CCD's) have been broadly applied in scanners and digital cameras to quantifiably measure luminance, illuminance and color coordinates of light sources or any illuminated object. In a charge coupled device camera, a light sensitive charge coupled device and a lens are incorporated. The image of an object or a light source to be measured is then focused onto the charge coupled device by the lens.
Since such charge coupled devices are very sensitive to light, stray light emitted from a region other than the object or the light source is often measured or recorded thereby. The sources of such stray light include the following:    1. The lens: Fresnel reflection or surface imperfections.    2. The interior walls of the lens barrel.    3. The glass window typically placed in front of the charge coupled device.    4. Reflected light from the surface of the charge coupled device passing back through the glass window and lens, from where it may scatter back onto the charge coupled device.    5. Reflected light from the surface of the charge coupled device that may scatter onto the inside walls of the charge coupled device chamber; and    6. External scattered light (for example, auto headlamp on curved wall, or light from one portion of wall scattered to another portion of wall and then into camera).
Such stray light ultimately returns, i.e., lands back on the charge coupled device in some unknown location that is not necessarily related to the corresponding position on the object from which it was emitted. For example, if a camera is capturing an image of a display with a checkerboard pattern of light and dark squares, some light will land on the charge coupled device corresponding to the focused position of a dark square, even though no light was actually emitted therefrom.
This stray light causes an inaccuracy when a charge coupled device is used to make quantifiable color, illuminance, or luminance measurement. While measuring illuminance or luminance, if the object being measured is highly non-uniform such as the checkerboard pattern or an automotive headlamp beam pattern, the stray light will tend to cause the measured light levels in darker regions of the object to be higher than they actually are.
One well-known method for accomplishing color measurement is to image a test area or a light source through two or more CIE matching filters. Again, if the object is significantly non-uniform while being viewed through one or more of the filters, stray light may contribute to inaccurate relative light levels for a specific filter. Since color coordinates are computed from the relative light levels of the matching filters, if the measurement is too high at a given location for a specific filter due to stray light, the particular color coordinate is not accurate at that particular location.
As an example of the above color measurement procedure, if an object with a red region and a blue region is measured by a camera with a charge coupled device, then as the blue measurement is performed, only light from the blue region of the object should land on the corresponding area of the charge coupled device. But due to stray light, some of the blue light may land on the charge coupled device corresponding to the red region of the object. This results in an inaccurate color coordinate for the red region because the measured value for the blue filter is too high.